The paper reported that the married MP had struck up an online relationship with a woman on Twitter calling herself "Sophie Wittams", whose account described her as a "20-something Tory PR girl" and eventually convinced him to send the explicit image.

Mr Newmark, 57, resigned on Saturday evening, the day before the Tory party conference in Birmingham, after learning what had happened and what was about to be published.

But on Sunday, questions surfaced about whose pictures the now-deleted Wittams Twitter account had used to encourage him to sext the image of himself.

The subterfuge was reportedly carried out by a freelance journalist rather than either of the staff journalists whose bylines appeared on the story.

If the story is subject to a formal complaint, it could be a test for Ipso, the new press body that replaced the Press Complaints Commission earlier this month that the Sunday Mirror has signed up to.

Its code for editors says: "Engaging in misrepresentation or subterfuge, including by agents or intermediaries, can generally be justified only in the public interest and then only when the material cannot be obtained by other means."

Lloyd Embley, the editor-in-chief of the paper and its daily counterpart, The Mirror, defended the story.

He was asked by Sue Llewellyn, a journalist and social media consultant, about the story, which she called "unethical".

He implied the paper could rely on a public interest defence for the methods used, saying part of Mr Newmark's ministerial brief was to "attract young women to the Tories".

Meanwhile, someone else set up new a Twitter account, @sophie_wittams, which posted a single tweet saying: "Pay no attention to this. It's basically just an entrapment account set up by the Sunday Mirror. Oldest trick in the book. Move along."

Pay no attention to this. It's basically just an entrapment account set up by the Sunday Mirror. Oldest trick in the book. Move along.